The present invention is directed to a method and apparatus for controlling traffic in a cellular telecommunications network, and more particularly, for controlling low capacity mobile assisted handoff in a cellular telecommunications network.
Conventional cellular mobile radio telephone systems include a land system and many mobile stations. The land system consists of at least one mobile switching center and several radio base stations which are distributed over a geographical area. Each radio base station serves a cell, i.e., a part of the geographical area, by sending and receiving information to and from mobile stations over radio channels. The mobile switching center, connected on one side to the public switching telephone network and on the other to the base stations, performs most of the control functions required for calls to and from the mobile stations in the geographical area. Because the cells are relatively small, mobile stations often travel between a number of different cells. The process of switching the established call from a home base station in one cell to a target base station in another cell is known as handoff.
In existing analog Frequency Division Multiple Access (FDMA) cellular systems, a base station determines the need for a handoff of mobile stations it is handling based on periodic measurements of the signal strength and signal quality of the uplink (mobile station to base station) voice channel signals received from each mobile station engaged in a call. If the measured signal strength or signal quality is below a predetermined level, that home base station sends a handoff request to the mobile switching center. The mobile switching center queries neighboring base stations for reports of the previously measured signal strength of signals on the voice channel currently being used by the mobile station. A neighboring base station is defined as any base station in the geographical vicinity of the serving base station that detects or can detect the signal strength of communications between the serving base station and one or more mobile stations served by the serving base station. The mobile switching center selects the neighbor base station reporting the strongest signal, provided that signal is above a threshold, and transmits appropriate commands to that target base station and to the mobile station via the home base station to tune to a specified new voice channel. One example of a neighbor-assisted handoff procedure is disclosed in U.S. Pat. No. 5,175,867, commonly-assigned with the present invention.
In dual-mode systems, as specified in the EIA/TIA IS-54 standard, handoff may also be implemented using Mobile-Assisted Handoff (MAHO). Using this procedure, a mobile station may be ordered by the land system to measure and report signal strength and other parameters of the digital radio channel emitted by the home base station, and the neighbor base stations. This enables handoff requests by a home base station to be based not only on the signal strength and other parameters of the uplink signal received from the mobile station, but also on the down link signal parameters detected by the mobile station of the home and neighbor base stations. However, when performing a handoff from an analog to a digital channel, MAHO cannot be used because the mobile station cannot report signal strength or other signal parameter measurements over an analog channel.
In purely digital cellular systems, mobile-assisted handoff may always be used. For a more detailed description of mobile-assisted handoff, reference is made to U.S. Pat. No. 5,042,082.
One of the disadvantages that results when these handoff methods are used is that a large amount of network resources are consumed during the handoff process. In particular, in digital networks, the mobile station continually reports to the land system signal strength measurement values for its own current connection as well as signal strength measurement values of the messages the mobile station receives on the broadcast channel of the neighboring cells. The measurements from the mobile station together with signal strength measurements from the base station are the basis for the evaluation of the measurements by the land system.
This handoff process is illustrated in FIG. 1, from the network or land system side. In particular, the network performs the measurement collection (10) from the mobile stations and from the base stations in the network. The network then performs the measurement evaluation (12) to determine whether or not handoff should occur (14) and which base station should be selected to receive the handoff. This evaluation process utilizes a large part of the resources used in the handoff process. In particular, due to the large amount of data required to be processed, system processors are tied up with such evaluations and therefore are unavailable for other functions.